musicmouse/slint-frontend/src/ws.rs

//! The state websocket, with reconnect-and-reseed.
//!
//! Ported from `web/src/hooks/usePlayerState.ts`. The backend pushes and never listens,
//! so this is a read loop and nothing else. Two details are load-bearing:
//!
//! * **Reseed on open, not just on connect.** A reconnect can have missed every change
//!   between the drop and the new socket, and the server only sends a snapshot to a
//!   client it already considers connected. So each successful open is followed by a
//!   `GET /api/state`.
//! * **Position is not on the event bus.** The server polls its player on a 0.5 s timer
//!   and sends `position` frames only while something is playing - so the absence of
//!   them is not a stall, and the UI interpolates between them rather than waiting.

use std::sync::mpsc::Sender;
use std::time::Duration;

use crate::api::{Client, PlayerState, ServerMessage};

/// What the UI thread hears from this connection. Wider than `ServerMessage` because
/// connectivity is itself a thing the screen shows.
pub enum Update {
    Connected,
    Disconnected,
    State(Box<PlayerState>),
    Position {
        position: f64,
        duration: f64,
    },
    /// The library was rescanned: re-fetch it, and drop the cover thumbnails, since a
    /// rescan is exactly when the art behind an unchanged URL gets rewritten.
    LibraryChanged,
}

/// Matches the web client's retry interval. Short enough that the screen comes back
/// before a child gives up on it, long enough not to spin while the backend restarts.
const RETRY: Duration = Duration::from_millis(1500);

pub fn spawn(client: Client, updates: Sender<Update>) -> std::thread::JoinHandle<()> {
    std::thread::Builder::new()
        .name("ws".into())
        .spawn(move || loop {
            match connect_once(&client, &updates) {
                Ok(()) | Err(()) => {}
            }
            // `send` failing means the UI is gone, which is the one reason to stop.
            if updates.send(Update::Disconnected).is_err() {
                return;
            }
            std::thread::sleep(RETRY);
        })
        .expect("spawning the websocket thread")
}

fn connect_once(client: &Client, updates: &Sender<Update>) -> Result<(), ()> {
    let (mut socket, _) = tungstenite::connect(client.websocket_url()).map_err(|_| ())?;

    if updates.send(Update::Connected).is_err() {
        return Err(());
    }
    // Reseed: see the note at the top of this module.
    if let Ok(state) = client.state() {
        if updates.send(Update::State(Box::new(state))).is_err() {
            return Err(());
        }
    }

    loop {
        let message = match socket.read() {
            Ok(message) => message,
            Err(_) => return Err(()),
        };
        let text = match message {
            tungstenite::Message::Text(text) => text,
            tungstenite::Message::Close(_) => return Ok(()),
            // Ping/Pong are answered by tungstenite itself on the next write; binary
            // frames are not part of this protocol.
            _ => continue,
        };
        let parsed: ServerMessage = match serde_json::from_str(&text) {
            Ok(parsed) => parsed,
            // A frame this build cannot read is not a reason to drop the connection -
            // the next one is probably fine.
            Err(_) => continue,
        };
        let update = match parsed {
            ServerMessage::State { state } => Update::State(Box::new(state)),
            ServerMessage::Position { position, duration } => {
                Update::Position { position, duration }
            }
            ServerMessage::Library => Update::LibraryChanged,
            ServerMessage::Unknown => continue,
        };
        if updates.send(update).is_err() {
            return Err(());
        }
    }
}